The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Wushao Wen wrote:
> 
> Yes. Shahram is right. In fact, the major task for RSVP is to reserve
> the necessary resource along the path for a connection. Explicit
> routing is not a  concern of the RSVP. RSVP does not change the basic
> packet routing mechanism--analyze the packet header and then do
> destination-based routing to select the next hop.

For straight RFC-2205 RSVP, yes.  RSVP is not a routing protocol.  It
establishes reservations along whatever path the local routing tables
will forward similarly-addressed data.

With RSVP-TE (draft-ietf-mpls-rsvp-lsp-tunnel-*), however, this
changes.  
The presence of an explicit route object forces RSVP to reserve
bandwidth along the specified route instead of where the local routing
table might otherwise dictate.  And it must make certain that data
packets for the session follow the reserved route instead of the route
that the routing tables would otherwise use.  (And when these data
packets are classified by an MPLS label, we call the result an LSP.)

In other words, while straight RSVP does not change the basic packet
forwarding mechanism, RSVP-TE definitely does.

-- David